The present invention is generally related to telecommunications apparatus, and more particularly to virtual circuits in telecommunications apparatus.
Voice carrying connection standards such as T1 and T3 multiplexed digital channels are well known. A T1 connection carries 24 standard voice channels and a T3 connection carries 28 T1 connections. T1 and T3 connections can also carry video signals and computer data. T1 and T3 are synchronous systems in which each individual voice connection has periodic time slots within which to transmit voice carrying data. Asynchronous systems, such as Asynchronous Transfer Mode ("ATM") networks, are also known. ATM networks can carry different types of data such as voice, video and computer data. However, while ATM systems are capable of transmitting voice data, the voice data is transmitted asynchronously.
Under some circumstances it is desirable to integrate synchronous networks and ATM networks. For example, voice data could be received by a first ATM switch from a "circuit" in a first synchronous network, propagated asynchronously to a second ATM switch through a "virtual circuit," and subsequently transmitted over a circuit in another synchronous network. In order to prevent gaps from occurring in the second synchronous network the second ATM switch must maintain a sufficient reserve of voice data units in a "playout buffer" to fill each time slot allocated to the voice connection in the second synchronous network with voice data. However, if the voice data units must be transmitted through many ATM switches, the amount of memory required to maintain a sufficient reserve of voice data units in the playout buffer can become prohibitively large. More particularly, as the number of intermediate ATM switches increases, the effect on variable transmission delay at each subsequent downstream ATM switch in the virtual circuit is cumulative.